You are unable to sign in to your Nintendo Account on Nintendo Switch and receive the error code 2124-5002.
This error indicates that there is likely a temporary service outage related to Nintendo Accounts.
What to Do:
Wait a while and try again later.
Situation Not Resolved:
If the issue persists, visit https://www.nintendo.co.jp/netinfo/en_AU/index.html and check for any posted service maintenance or outages.
If you would like further assistance with the Nintendo Switch console, please contact Nintendo Customer Service.